US8495870B2 - Work machine - Google Patents

Abstract

Disclosed is a work machine having a traveling apparatus and a work implement driven by an engine. A hydraulic system mounted on the work machine includes an HST pump comprising a swash plate, variable displacement pump driven by the engine, an HST motor connected in a closed circuit to the HST pump and a pair of speed-changing oil passageways, the HST motor being driven by an amount of oil supplied by the HST pump, thereby to drive the traveling apparatus, a main pump driven by the engine, the main pump supplying pressure oil to the work implement, a pilot pump driven by the engine, a swash plate positioning circuit configured to effect positioning of a swash plate of the HST pump with pilot oil supplied from the pilot pump, and a bleed circuit configured to drain, via a throttle, a portion of the pilot oil supplied to the swash plate positioning circuit.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a work machine such as a truck loader, a skid loader, etc. in which a traveling apparatus is driven via an HST (hydrostatic transmission) configured to drive an HST motor by an HST pump comprising a swash plate, variable displacement pump driven by an engine.

2. Description of the Related Art

As an example of a work machine including a traveling apparatus driven via an HST, there is known a machine including an HST pump comprising a swash plate, variable displacement pump, an HST motor connected in a closed circuit to this HST pump and a pair of speed-changing oil passageways and configured to be driven by discharge oil from the HST pump to drive a traveling apparatus, a charge circuit for supplementing an amount of oil from a charge pump to the speed-changing oil passageways, a charge relief valve for determining the circuit pressure of the charge circuit, a flushing valve for allowing escape of a portion of the work oil of the low-pressure side of the speed-changing oil passageways, a flushing relief valve incorporated in a flushing relief oil passageway for draining oil from the flushing valve, and a servo cylinder for controlling a swash plate of the HST pump so as to vary the discharge capacity of the HST pump according to the discharge rate from the charge pump (see JP 6-058411A).

In the case of a work machine configured such that its traveling apparatus is driven via an HST, if a significant load is applied to the HST motor during traveling, this load will be transmitted to the engine via the HST pump, so the rotational speed of the engine will be reduced, thus presenting the risk of stall of the engine. On the other hand, in the case of the construction described in JP 6-058411A, there is provided an anti-stall feature (anti-stall function). With this feature, when the rotational speed of the engine decreases, the rotational speed of the charge pump decreases also to reduce the discharge amount of this charge pump, whereby a control pressure of the servo cylinder is reduced, so that the swash plate angle of the HST pump is automatically adjusted so as to reduce the rotational speed of the HST pump. As a result, the load to the engine is reduced, thus providing an anti-stall feature (anti-stall function) for preventing engine stall.

In JP 6-058411A, as a method of improving the anti-stall feature for allowing the control pressure of the servo cylinder to drop more speedily in response to reduction in the rotational speed of the engine, it is conceivable to increase the amount of escape of the oil from the flushing relief valve or to increase the amount of escape of the oil from the charge relief valve. However, if the amount of the escape of the oil from the flushing relief valve is increased, due to the corresponding increase of the oil introduced to the flushing valve, e.g. malfunction of this flushing valve may occur. Further, there is a limit in increasing the amount of escape of the oil from the charge relief valve. Therefore, there is a limit in improvement of the anti-stall feature with these methods.

Further, in the case of an arrangement wherein the rotational speed of the engine is detected and a controller effects a control so that the control pressure of the swash plate of the HST pump is dropped speedily in response to application of excessive load to the engine, this will result in great complexity of the construction, thus inviting cost increase.

In view of the above-described problems, the object of the present invention is to provide a work machine that allows improvement of the anti-stall feature with simple arrangement.

SUMMARY OF THE INVENTION

The above-noted object is fulfilled according to an aspect of the present invention as under:—

A work machine having a traveling apparatus and a work implement driven by an engine,

wherein a hydraulic system mounted on the work machine comprises:

• • an HST pump comprising a swash plate, variable displacement pump driven by the engine,
  • an HST motor connected in a closed circuit to the HST pump and a pair of speed-changing oil passageways, the HST motor being driven by an amount of oil supplied by the HST pump, thereby to drive the traveling apparatus,
  • a main pump driven by the engine, the main pump supplying pressure oil to the work implement,
  • a pilot pump driven by the engine,
  • a swash plate positioning circuit configured to effect positioning of a swash plate of the HST pump with pilot oil supplied from the pilot pump, and
  • a bleed circuit configured to drain, via a throttle, a portion of the pilot oil supplied to the swash plate positioning circuit.

With the above-described characterizing arrangement of the present invention, with the provision of a bleed circuit configured to drain, via a throttle, a portion of the pilot oil supplied to the swash plate positioning circuit, the flow amount of the pilot oil to the swash plate positioning circuit is speedily decreased in response to reduction in the rotational speed of the engine, so that the pilot pressure for controlling the swash plate of the HST pump can be reduced speedily. Hence, it has become possible to provide a work machine which allows improvement in the anti-stall feature with a simple arrangement.

According to one preferred embodiment of the present invention, the hydraulic system further comprises: a charge relief valve for determining the circuit pressure of a charge circuit for supplementing an amount of oil from the pilot pump to the speed-changing oil passageways, and a flushing throttle incorporated in a flushing relief oil passageway for draining oil from a flushing valve configured to allow escape of a portion of work oil in the low-pressure side oil speed-changing passageway of the speed-changing oil passageways.

In the above arrangement, preferably, the flushing relief oil passageway incorporates a flushing relief valve disposed between the flushing valve and the flushing throttle.

According to a further preferred embodiment, the HST motor comprises a swash plate, variable displacement motor switchable between a first speed state and a second speed state, according to the positioning of the swash plate by pilot oil, and for switching over the HST motor from the first speed state to the second speed state, there is provided a second bleed circuit for draining, through a throttle, a portion of pilot oil supplied from the pilot pump.

According to a still further preferred embodiment, the work implement comprises a bucket capable of effecting a scooping/dumping action; and the hydraulic system includes a bucket cylinder for causing the bucket to effect a scooping/dumping action and a bucket control valve for controlling the bucket cylinder; and wherein in a hydraulic circuit at a scooping position where the bucket is caused to effect a scooping action by the bucket control valve, there is provided a bucket bleed circuit for draining, through a throttle, a portion of the pressure oil supplied from the main pump to the bucket cylinder.

With the above-described arrangement, when the work machine is advanced to plunge the bucket into an amount of earth/sand or the like and the bucket is caused to effect a scooping action, the load applied to the hydraulic pump as the driving source of the traveling apparatus is transmitted to the engine thereby to reduce the rotational speed of the engine. In response to this, the discharge amount of the main pump driven by the engine is reduced, so that the ratio of the amount of leak from the bucket bleed circuit relative to this discharge amount is increased correspondingly. Then, the pressure of the pressure oil supplied from the main pump to the bucket cylinder for causing the bucket to effect the scooping action is dropped, thus alleviating the load applied to the main pump. With this, it becomes possible to prevent engine stall which would occur otherwise in such case as high load is applied to the traveling apparatus and the bucket simultaneously.

In the above arrangement, the following construction may be added advantageously.

At the scooping position of the bucket control valve, the pressure oil from the main pump is supplied to a rod side oil chamber of the bucket cylinder.

Further, the bucket control valve includes two pump ports for inputting the pressure oil from the main pump, two cylinder ports for supplying/discharging pressure oil to/from the rod side oil chamber and a bottom side oil chamber of the bucket cylinder, and a tank port communicated to a tank; and at the scooping position of the bucket control valve, one of the pump ports becomes communicated to the tank port via the bucket bleed circuit and also the other pump port becomes communicated to the cylinder port connected to the rod side oil chamber of the bucket cylinder.

Further, the bucket control valve includes a return oil passageway for returning the oil discharged from the bottom side oil chamber of the bucket cylinder to the upstream side of the throttle of the bucket bleed circuit via a check valve, at the scooping position.

Further, the work implement includes the bucket at a leading end of an arm which is pivoted up/down by a lift cylinder operable by the pressure oil from the main pump; and

the hydraulic system further comprises:

• • a lift cylinder for pivoting up/down the arm with the pressure oil from the main pump,
  • an arm control valve for controlling the lift cylinder;
  • a work implement operating apparatus for operating the arm control valve and the bucket control valve;
  • pressure reducing means for reducing a primary pressure of the work implement operating apparatus for preventing full stroke action of the arm control valve, in response to drop in rotational speed of the engine;

wherein said arm control valve and said bucket control valve are disposed in series in a work implement supplying oil passageway for supplying the pressure oil discharged from the main pump, with the arm control valve being disposed on the upstream side of the bucket control valve.

According to a still further preferred embodiment, there is provided a traveling operating apparatus for controlling pilot oil in the swash plate positioning circuit;

there is provided a work implement operating apparatus for controlling the pilot pressure to a control valve for controlling the work implement;

a hydraulic passageway is branched so as to supply the discharge oil from the pilot pump to the traveling operating apparatus and the work implement operating apparatus, and in a hydraulic circuit between this branching point and the traveling operating apparatus, there is provided a pressure compensating valve for ensuring a primary pressure for the work implement operating apparatus, and

on the upstream side of the pressure compensating valve, there is provided a work implement bleed circuit for draining, through a throttle, a portion of the pressure oil discharged by the pilot pump and supplied to the work implement operating apparatus.

With the above-described arrangement, when the work machine is advanced to plunge the work implement into earth/sand or the like, the load applied to the hydraulic pump acting as the driving source of the traveling apparatus is transmitted to the engine thereby to reduce the rotational speed of the engine.

In response to this reduction in the rotational speed of the engine, the discharge amount of the pilot pump decreases. So that, the ratio of the amount of leak from the bleed circuit relative to the discharge amount of the pilot pump is increased. With this, it becomes possible to allow the primary pressure of the work implement operating apparatus to drop below a pressure set by the pressure compensating valve.

And, as the primary pressure of the work implement operating apparatus is caused to drop below a pressure set by the pressure compensating valve, the control valve for controlling the work implement becomes unable to effect full stroke action. If the control valve for controlling the work implement does not effect full stroke action, a portion of the pressure oil supplied to the work implement from the main pump to the work implement via the control valve is drained to the drain oil passageway, so that the pressure of the pressure oil from the main pump drops, and the load applied to this main pump is alleviated.

With the above, it becomes possible to prevent engine stall e.g. when high load is applied to the traveling apparatus and the work implement simultaneously.

Advantageously and alternatively, instead of the work implement bleed circuit, on the upstream side of the work implement operating apparatus, there may be provided a pressure reducing valve capable of pressure adjustment so as to allow drop of the primary pressure of the work implement operating apparatus.

According to a still further preferred embodiment, there is provided a traveling operating apparatus for controlling the pilot oil in the swash plate positioning circuit;

the HST pump includes a pair of pressure receiving portions for receiving the pilot pressure from the traveling operating apparatus via a shock relieving throttle, and the angle of the swash plate is controlled by pressure difference between these pressure receiving portions, in the course of which in association with reduction in the rotational speed of the engine, the primary pressure of the traveling operating apparatus drops and the swash plate of the HST pump returns to the neutral side, thus preventing engine stall; and

there is provided an escape oil passageway having one end thereof communicated to a primary side hydraulic passageway of the traveling operating apparatus and the other end thereof communicated to a hydraulic passageway between the shock relieving throttle and one of the pressure receiving portions, and the escape oil passageway incorporates a check valve which is opened when the pressure of the one pressure receiving portion to which the other end of the escape oil passageway is communicated is greater than the primary pressure of the traveling operating apparatus.

With this arrangement, when the pressure of the pressure receiving portions to which the other end of the escape oil passageway is communicated is smaller than the primary pressure of the traveling operating apparatus, the check valve is closed, so that the pilot pressure is allowed to communicate via the shock relieving throttle in the hydraulic passageway between the traveling operating apparatus and the pressure receiving portions, thus preventing sudden speed change.

And, when an excessive load is applied suddenly to the engine, thus resulting in sudden drop in the primary pressure of the traveling operating apparatus and the pressure of the pressure receiving portions to which the other end of the escape oil passageway is communicated is greater than the primary pressure of the traveling operating apparatus, the check valve is opened, so that the pressure of the pressure receiving portion is allowed to escape through the escape oil passageway to the primary side of the traveling operating apparatus, and the swash plate of the HST pump returns to the neutral side speedily.

Therefore, sudden speed change is prevented by the shock relieving throttle and at the same time, the response performance of the anti-stall feature can be improved by the simple arrangement of providing a check valve in the escape oil passageway.

Further and other features and advantages resulting therefrom will become apparent upon reading the following detailed description with reference to the accompanying drawings.

In the following description, the respective embodiments will be explained with taking a truck loader as an example of the work machine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-9 show a work machine (truck loader) according to a first embodiment of the present invention; in which,

FIG. 1 is a hydraulic circuit diagram showing a hydraulic system of the traveling line,

FIG. 2 is a hydraulic circuit diagram of a hydraulic system of the work machine,

FIG. 3 is a hydraulic circuit diagram of the implement line,

FIG. 4 is an overall side view of a work machine,

FIG. 5 is a side view in section showing a portion of the work machine with its cabin being elevated,

FIG. 6 is a graph showing an example of pressure characteristics of anti-stall function of the hydraulic system shown inFIG. 1,

FIG. 7 is a graph showing a further example of pressure characteristics of anti-stall function of the hydraulic system shown in FIG.1,

FIG. 8 is a graph showing pressure characteristics of anti-stall function relating to a comparison example,

FIG. 9 is a graph showing pressure characteristics of anti-stall function relating to a further comparison example,

FIGS. 10-15 show a work machine (truck loader) according to a second embodiment of the present invention; in which,

FIG. 10 is a hydraulic circuit diagram showing a hydraulic system of the work machine,

FIG. 11 is a hydraulic circuit diagram showing a hydraulic system of the traveling line,

FIG. 12 is a schematic hydraulic circuit diagram showing the hydraulic system of the traveling line,

FIG. 13 is a hydraulic circuit diagram showing a hydraulic system of the implement line,

FIG. 14 is a schematic hydraulic circuit diagram showing the hydraulic system of the implement line, and

FIG. 15 is a schematic hydraulic circuit diagram showing a further hydraulic system of the implement line.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSFirst Embodiment

First, with reference toFIGS. 1-9, a first embodiment will be described.

InFIG. 4 andFIG. 5, a work machine1 (truck loader) relating to the present invention includes amachine body2, a work implement3 mounted on themachine body2, and a pair of right/left traveling apparatuses4 supporting themachine body2, and a cabin5 (driver protecting apparatus) is mounted at an upper portion on themachine body2, with an offset to the forward side.

Themachine body2 is formed of iron plates or the like, and includes abottom wall5, a pair of right/left side walls7, afront wall8 and support frames9 provided at the rear portions of the right/leftrespective side walls7. Between theside walls7, there is formed an upwardly opened space, and at the rear end of thismachine body2, there is provided an openable/closable lid member10 for closing the rear end opening between the right/left support frames9.

Thecabin5 is mounted with its front lower end in contact with anupper edge portion8aof thefront wall8 of themachine body2 and has a vertical intermediate portion of the rear face thereof supported to be pivotable about a right/left orientedsupport shaft12. By pivoting thecabin5 about thesupport shaft12 upwardly, e.g. a maintenance operation of the inside of themachine body2 is made possible.

Inside thecabin5, there is provided a driver'sseat13. And, on one of the right/left sides (e.g. the left side) of this driver'sseat13, there is provided a travelingoperating apparatus14 for operating the traveling apparatuses4 and on the other side of the right/left sides (e.g. the right side) of the driver'sseat13, there is provided an implementoperating apparatus15 for operating the work implement3.

The upper side of thecabin5 is closed with a roof and right/left side faces thereof are closed with side walls defining a number of rectangular holes. The rear upper portion of thecabin5 is closed with a rear glass sheet and at a front/rear center portion of the bottom side is closed with a bottom wall, so that the cabin is formed like a box with its front side open and the front side constitutes an entrance/exit for the driver.

Each one of the right/left traveling apparatuses4 is constructed as a crawler type traveling apparatus including a pair of front/rear drivenwheels16, adrive wheel17 disposed upwardly and between the front/rear drivenwheels16, with an offset toward the rear side, a plurality oftrack rollers18 disposed between the front/rear drivenwheels16, and an endless belt likecrawler belt19 entrained about and across these front/rear drivenwheels16, thedrive wheel17 and thetrack rollers18.

The front/rear drivenwheels16 and thetrack rollers18 are mounted to atrack frame20 fixedly attached to themachine body2 to be rotatable about horizontal axes and thedrive wheel17 is mounted to a rotary drum of a hydraulically driven travelingmotor21L,21R (wheel motors) mounted to thetrack frame20. In operation, as thedrive wheel17 is driven to rotate about a right/left axis by the travelingmotor21L,21R, thecrawler belt19 is driven to run in circulation in the peripheral direction, whereby thework machine1 travels forwardly or in reverse.

The work implement3 includes a pair of right/left arms22, and a bucket23 (implement) attached to the leading ends of thearms22.

The right/left arms22 are disposed on the right/left opposed sides of themachine body2 and thecabin5, with the right/left arms22 being interconnected via an interconnecting member at their front side intermediate portions thereof.

Each one of the right/left arms22 has its leading end side vertically pivotable at positions forwardly of themachine body2, and its base end side (rear end side) supported to be vertically pivotable to the rear upper portion of themachine body2 via afirst lift link24 and asecond lift link25.

Further, between the base portion of each right/left arm22 and the rear lower portion of themachine body2, there is provided alift cylinder26 comprising a double-action type hydraulic cylinder. In operation, as the right/left lift cylinders26 are extended/contracted at one time, the right/left arms22 are pivoted up/down.

To the leading end of each right/left arm22, a mountingbracket27 is connected to be pivotable about a right/left axis, and to these right/left mountingbrackets27, the rear face of thebucket23 is attached.

Further, between the mountingbracket27 and the leading end side intermediate portion of thearm22, there is interposed atilt cylinder28 comprising a double action type hydraulic cylinder. In operation, in association with expansion/contraction of thistilt cylinder28, thebucket23 is pivoted (thebucket23 effects a scooping/dumping action).

Thebucket23 is detachably attached to the mountingbrackets27. When thebucket23 is detached and any one of various kinds of attachments (a hydraulically driven implement) is attached to the mountingbrackets27, various kinds of work other than excavating (or other type of excavating work) are made possible.

At a rear portion on thebottom wall6 of themachine body2, anengine29 is mounted and at a front portion on thebottom wall6 of themachine body2, there are mounted afuel tank30 and awork oil tank31.

Forwardly of theengine29, there is provided ahydraulic drive apparatus32 for driving the right/left traveling motors21L,21R, and forwardly of thishydraulic drive apparatus32, first through third pumps P1, P2, P3 are provided. At a front/rear intermediate portion of theright side wall7 of themachine body2, there is provided a control valve33 (hydraulic controlling apparatus) for the work implement3.

Next, with reference toFIGS. 1-3, the hydraulic system of thework machine1 will be described.

The first through third pumps P1, P2, P3 each is comprised of a fixed displacement type gear pump driven by the power of theengine29.

The first pump P1 (main pump) is used for driving a hydraulic actuator of an attachment attached to the leading end of thelift cylinder26 thetilt cylinder28 or thearm22.

The second pump P2 (pilot pump or charge pump) is used mainly for supplying a control signal pressure (pilot pressure).

The third pump P3 (sub pump) is used for increasing a flow rate of work oil to a hydraulic actuator of a hydraulically driven attachment attached to the leading end of thearm22 in case this hydraulic actuator requires a large capacity.

The travelingoperating apparatus14 includes a forward travelingpilot valve36, a reverse travelingpilot valve37, a right turningpilot valve38, a leftturning pilot valve39, a (single) travelinglever40 common to thesepilot valves36,37,38,39 and first throughfourth shuttle valves41,42,43,44.

A main supply passageway (a) for feeding discharge oil (pilot oil) of the second pump P2 is branched at a branching point (b) into a first supplying passageway (c), a second supplying passageway (d) and a third supplying passageway (e), and the third supplying passageway (e) is further branched into a fourth supplying passageway (f) and a fifth supplying passageway (g).

The pilot oil of the first supplying passageway (c) (the discharge oil from the second pump P2) can be supplied to therespective pilot valves36,37,38,39 of the travelingoperating apparatus14 with magnetic excitation of a travelinglock valve46 comprising an electromagnetic type two-position switch valve. In response to demagnetization of this travelinglock valve46, the pilot oil of the first supplying passageway (c) cannot be supplied to therespective pilot valves36,37,38,39 of the travelingoperating apparatus14, so that the travelingoperating apparatus14 become inoperable.

Each right/left traveling motor21L,21R includes anHST motor47 comprising a swash plate, variable displacement type axial motor switchable in two, high/low speeds, a swashplate switching cylinder48 for speed-changing theHST47 in the two, high/low speeds by switching over the angle of the swash plate, abrake cylinder49 for braking anoutput shaft47aof the HST motor47 (output shaft47aeach of the travelingmotors21L,21R), a flushingvalve50, aflushing relief valve51 and aflushing throttle67.

The swashplate switching cylinder48, when no pressure oil is being applied thereto, renders theHST motor47 into the first speed condition and renders, when pressure oil is being applied thereto, theHST motor47 into the second speed condition.

Whether to apply the pressure oil to this swashplate switching cylinder48 or not is determined by acylinder switching valve68 comprising a pilot operation type two-position switching valve and thiscylinder switching valve68 can be switched over by a two-speed switching valve45 comprising an electromagnetic type switching valve.

In more particular, when the two-speed switching valve45 is demagnetized so that the second supplying passageway (d) is shut by this two-speed switching valve45, no pilot pressure is applied to thecylinder switching valve68 and also no pressure oil is applied to the swashplate switching cylinder48, so that theHST motor47 is under the first speed condition. And, when the two-speed switching valve45 is excited by operating means, the two-speed switching valve45 is switched over so as to apply the pilot pressure of the second supplying passageway (d) (the discharge oil from the second pump P2) to thecylinder switching valve68, whereby thecylinder switching valve68 is switched over to apply the pressure oil to the swashplate switching cylinder48, so that theHST motor47 is rendered into the second speed condition.

Thus, the switchover of the swash plate of theHST motor47 is effected such that theHST motor47 is switched over from the first speed condition to the second speed condition with utilizing the pilot oil from the second pump P2.

Incidentally, in the present embodiment, the swashplate switching cylinder48 is configured to receive the high pressure side work oil from speed-changing oil passageways (h, i) to be described later. Instead, it may be arranged such that the swashplate switching cylinder48 is operated directly by the pilot oil from the second pump P2.

Thebrake cylinder49 brakes theoutput shaft47aof theHST motor47 with an urging force of a spring, and as abrake release valve52 comprising an electromagnetic type two-position switching valve is magnetized, the pilot oil of the fourth supplying passageway (e) (the discharge oil from the second pump P2) is applied to thisbrake cylinder49, thus releasing the braking of theoutput shaft47aof theHST motor47.

To the travelinglock valve46 and thebrake release valve52, demagnetizing signals are transmitted simultaneously by means of e.g. a lock lever which is operated when a driver gets out of thecabin5 and magnetizing signals are transmitted simultaneously thereto e.g. by a release switch.

The flushingvalve50 and theflushing relief valve51 will be described later herein.

Thehydraulic drive apparatus32 includes a left travelingmotor driving circuit32A (left driving circuit) and a right travelingmotor driving circuit32B (right driving circuit). Each drivingcircuit32A,32B includes anHST pump53 connected to theHST motor47 of the corresponding travelingmotor21R,21L with a pair of speed-changing oil passageways (h, i), a highpressure relief valve54 for releasing the pressure to the low pressure side of the speed-changing oil passageways (h, i) when the pressure of the high pressure side of the speed-changing oil passageways (h, i) exceeds a predetermined pressure, and a charge circuit (j) for supplementing pressure oil from the second pump P2 to the low pressure side oil passageway (h, i) via thecheck valve55.

Components of thehydraulic drive apparatus32 are incorporated within a housing.

The charge circuit (j) can receive the oil (discharge oil from the second pump P2) of the charge pressure supplying passageway (k) branched from the first supplying passageway (c) and connected to each charge circuit (j), and theleft driving circuit32A incorporates acharge relief valve56 for setting the circuit pressure each of the drivingcircuits32A,32B.

The second pump P2, in this embodiment, functions not only as a pilot pump for supplying pilot oil to therespective pilot valves36,37,38,39 of the travelingoperating apparatus14 and to the swashplate switching cylinder48 and thebrake cylinder49, but also as a charge pump for supplying oil to the charge circuit (j).

TheHST pump53 each of the drivingcircuits32A,32B functions not only as a swash plate, variable displacement type axial pump driven by the power of theengine29, but also as a pilot type hydraulic pump having a swash plate whose angle is changed by the pilot pressure, and includes a forward travelingpressure receiving portion53aand a reverse travelingpressure receiving portion53bto which the pilot pressure is applied. As the swash plate angle is changed by the pilot pressure applied to thesepressure receiving portions53a,53b, a direction and an amount of work oil discharged from theHST pump53 are changed, whereby the rotational output each of the travelingmotors21L,21R can be speed-changed in stepless manner in the direction for moving thework machine1 forwardly (forward rotation direction) or in the direction for moving thework machine1 in reverse (reverse rotation direction).

The flushingvalve50 each of the travelingmotors21L,21R is switched over by the pressure of the high pressure side of the speed-changing oil passageways (h, i) to connect the low pressure side of the speed-changing oil passageways (h, i) to the flushing relief oil passageway (m), and flushes a portion of the work oil of the low speed side oil passageway (h, i) via a flushing relief oil passageway (m) to the oil pan inside the housing each of the travelingmotors21L,21R so as to supplement an amount of work oil to the low pressure side of the speed-changing oil passageways (h, i). Incidentally, the oil in the oil pan inside the housing each of the travelingmotors21L,21R is returned to thework oil tank31 via a drain circuit (n).

The flushingrelief valve51 and theflushing throttle67 are incorporated in the flushing relief oil passageway (m) and theflushing relief valve51 is interposed between the flushingvalve50 and theflushing throttle67.

TheHST motor47 and the flushingvalve50, etc. each of the travelingmotors21L,21R, the drivingcircuits32A,32B and the pair of speed-changing oil passageways (h, i) together constitute a separate type HST (hydrostatic transmission).

The travelinglever40 of the travelingoperating apparatus14 is operable pivotally with inclination from the neutral position in the forward/reverse and right/left directions and along the oblique directions between the forward/reverse and right/left directions. As the travelinglever40 is pivotally operated; each of thepilot valves36,37,38,39 of the travelingoperating apparatus14 becomes operable to output therefrom the pilot pressure in proportion to the pivotal amount from its neutral position of the travelinglever40.

In more particular, as the travelinglever40 is operated to the front side (the arrow direction A1 inFIG. 1), the forward travelingpilot valve36 is operated so that pilot pressure is outputted from thispilot valve36 and this pilot pressure is applied to the forward travelingpressure receiving portion53aof theHST pump53 of theleft driving circuit32A via thefirst shuttle valve41 and applied also to the forward travelingpressure receiving portion53aof theright driving circuit32B via thesecond shuttle valve42, whereby theoutput shafts47aof the right/left traveling motors21L,21R are driven forwardly (forward rotation) at a speed in proportion to the pivotal amount of the travelinglever40, so that thework machine1 travels forward. Further, as the travelinglever40 is operated to the rear side (the arrow direction A2 inFIG. 1), the reverse travelingpilot valve37 is operated so that pilot pressure is outputted from thispilot valve37 and this pilot pressure is applied to the reverse travelingpressure receiving portion53bof theHST pump53 of theleft driving circuit32A via thethird shuttle valve43 and applied also to the reverse travelingpressure receiving portion53bof theHST pump53 of theright driving circuit32B via thefourth shuttle valve44, whereby theoutput shafts47aof the right/left traveling motors21L,21R are driven reversely (reverse rotation) at a speed in proportion to the pivotal amount of the travelinglever40, so that thework machine1 travels reverse.

Further, as the travelinglever40 is operated to the right side (the arrow direction A3 inFIG. 1), the right turningpilot valve38 is operated so that pilot pressure is outputted from thispilot valve38 and this pilot pressure is applied to the forward travelingpressure receiving portion53aof theHST pump53 of theleft driving circuit32A via thefirst shuttle valve41 and applied also to the reverse travelingpressure receiving portion53bof theHST pump53 of theright driving circuit32B via thefourth shuttle valve44, whereby theoutput shaft47aof theleft driving motor21L is rotated forwardly and also theoutput shaft47aof theright traveling motor21R is rotated in reverse, so that thework machine1 turns to the right side.

Further, as the travelinglever40 is operated to the left side (the arrow direction A4 inFIG. 1), the left turningpilot valve39 is operated so that pilot pressure is outputted from thispilot valve39 and this pilot pressure is applied to the forward travelingpressure receiving portion53aof theHST pump53 of theright driving circuit32B via thesecond shuttle valve42 and applied also to the reverse travelingpressure receiving portion53bof theHST pump53 of theleft driving circuit32A via thethird shuttle valve43, whereby theoutput shaft47aof theright driving motor21R is rotated forwardly and also theoutput shaft47aof theleft traveling motor21L is rotated in reverse, so that thework machine1 turns to the left side.

Still further, as the travelinglever40 is pivoted along the oblique direction, due to the pressure difference between the pilot pressures applied to the forward travelingpressure receiving portion53aand the reverse travelingpressure receiving portion53beach of the drivingcircuits32A,32B, the rotational direction and the rotational speed of theoutput shaft47aeach of the travelingmotors21L,21R are determined, such that thework machine1 will make a right turn or a left turn while traveling forwardly or in reverse. (More particularly, when the travelinglever40 is pivoted in the forward obliquely left direction, thework machine1 will turn left while traveling forward at the speed corresponding to the pivotal angle of the travelinglever40. When the travelinglever40 is pivoted in the forward obliquely right direction, thework machine1 will turn right while traveling forward at the speed corresponding to the pivotal angle of the travelinglever40. When the travelinglever40 is pivoted in the reverse obliquely left direction, thework machine1 will turn left while traveling reverse at the speed corresponding to the pivotal angle of the travelinglever40. When the travelinglever40 is pivoted in the reverse obliquely right direction, thework machine1 will turn right while traveling reverse at the speed corresponding to the pivotal angle of the travelinglever40.)

Further, theengine29 can be accelerated by an accelerator from the idling rotational speed to a rated rotational speed. When the rotational speed of theengine29 is increased, the rotational speed of theHST pump53 is increased, whereby the discharge amount of thisHST pump53 is raised, and the traveling speed is increased.

To the charge pressure supplying passageway (k), there is connected a bleed circuit69 (this will be referred to as “afirst bleed circuit69” hereinafter).

Thisfirst bleed circuit69 includes a bleed oil passageway69ahaving one end thereof connected to the charge pressure supplying passageway (k) and the other end thereof communicated to the oil pan of the housing of thehydraulic drive apparatus32, and further includes athrottle69bincorporated in this bleed oil passageway69a.

The oil of the oil pan of the housing of thehydraulic drive apparatus32 is returned to thework oil tank31 via the drain circuit (n).

The pilot oil discharged from the second pump P2 and supplied to the travelingoperating apparatus14 via the first supplying passageway (c) is supplied also to the charge circuit (j) via the charge pressure supplying passageway (k) and a portion thereof is drained by thefirst bleed circuit69 through thethrottle69bof thisbleed circuit69.

Incidentally, the oil drained via thefirst bleed circuit69 could be directly returned to thework oil tank31. However, as this oil is drained to the inside of the housing of the hydraulic drive apparatus32 (i.e. the housing of the HST pump53), cooling of theHST pump53, etc. is made possible. Further, in the hydraulic system, the flushingrelief valve51 can be omitted.

Further, the other end of the bleed oil passageway69acan be communicated to the relief oil passageway (o) for guiding the oil to be drained from thecharge relief valve56 to the oil pan of the housing of thehydraulic drive apparatus32.

With thework machine1 having the above-described arrangements, when e.g. thework machine1 is advanced to plunge thebucket23 into an amount of piled-up sand/earth or the like, a load will be applied to theHST motor47. In this, this load applied to theHST motor47 will be transmitted via theHST pump53 to theengine29, whereby the rotational speed of theengine29 will drop.

Then, the rotational speed of the second pump P2 will decrease, thus decreasing the discharge amount of this second pump P2 and the ratio of the oil leak from thefirst bleed circuit69 relative to this discharge amount of the second pump P2 will become larger. As a result, the pilot pressure outputted from the travelingoperating apparatus14 will drop speedily according to the reduction in the rotational speed of theengine29. With this, the swash plate angle of theHST pump53 will be automatically adjusted in a speedy manner so as to reduce the rotational speed, thus reducing the load applied to theengine29. As a result, the stall of theengine29 can be avoided effectively.

Even if thefirst bleed circuit69 were not provided, as shown inFIG. 8 andFIG. 9, due to the escape of the oil from theflushing throttle67 and the override characteristics of the charge relief valve56 (the escape of oil from the charge relief valve56), there could be provided the anti-stall characteristics for reducing the control pressure of the swash plate of theHST pump53 in response to reduction in the rotational speed of theengine29. But, the effect of such arrangement would be small.

In contrast, according to the present invention, in addition to the escape of the oil from theflushing throttle67 and the override characteristics of thecharge relief valve56, thanks to the leak of oil from thefirst bleed circuit69, as shown inFIG. 6 andFIG. 7, effective anti-stall characteristics (effective anti-stall performance) can be obtained.

FIGS. 6 through 9 are graphs of the pressure characteristics curves of the anti-stall feature, the horizontal axis representing the rotational speed of theengine29, the vertical axis representing the pilot pressure outputted from the travelingoperating apparatus14 to the HST pump53 (control pressure for the swash plate of the HST pump53).

FIGS. 8 and 9 are graphs of the pressure characteristics curves of the anti-stall feature, in case in the hydraulic system shown inFIG. 1, thefirst bleed circuit69 and theflushing relief valve51 are omitted therefrom. And,FIG. 8 shows the case of using a standardcharge relief valve56, whereasFIG. 9 shows the case of using acharge relief valve56 with an enhanced anti-stall performance.

In theseFIGS. 8 and 9, a portion Y on the right side relative to a flexion point X of the pressure characteristics curve of the anti-stall feature represents the condition when thecharge relief valve56 is opened (the condition when the oil is escaped from theflushing throttle67 and the charge relief valve56) and a portion Z on the left side relative to the flexion point X of the pressure characteristics curve of the anti-stall feature represents the condition when thecharge relief valve56 is closed (the condition when the oil is escaped only from the flushing throttle67).

In the cases shown inFIGS. 8 and 9, the flexion point X is located at a position of the rotational speed of theengine29 being low, so that the right side portion Y relative to the flexion point X has a small inclination and is straight. So, the drop of the pilot pressure relative to the drop in the rotational speed from the rated rotation (2400 rpm) of theengine29 is small. Therefore, the response of the pilot pressure to the drop in the rotational speed of theengine29 is slow.

Further, in the case of the pressure characteristics curve depicted inFIG. 9, as compared with the pressure characteristics curve depicted inFIG. 8, the right side portion Y relative to the flexion point X has a greater inclination, but this is still far from the ideal anti-stall pressure characteristics curve. Incidentally, in the pressure characteristics curve depicted inFIG. 9, unless a certain level of pilot pressure is ensured in the rated rotation (2400 rpm) of theengine29, control of the swash plate of theHST pump53 at the time of high load will be impossible. Therefore, this point W cannot be lowered excessively.

FIG. 6 shows the pressure characteristics curve in case in the hydraulic system shown inFIG. 1, the flushingrelief valve51 is omitted therefrom.

In the case of the one shown inFIG. 6, a portion Y on the right side relative to a flexion point X of the pressure characteristics curve of the anti-stall feature represents the condition when thecharge relief valve56 is opened (the condition when the oil is escaped from thefirst bleed circuit69, theflushing throttle67 and the charge relief valve56) and a portion Z on the left side relative to the flexion point X of the pressure characteristics curve of the anti-stall feature represents the condition when thecharge relief valve56 is closed (the condition when the oil is leaked from thefirst bleed circuit69 and the flushing throttle67).

In the case of the one shown in thisFIG. 6 too, like the ones shown inFIGS. 8 and 9, the right side portion Y relative to the flexion point X in the pressure characteristics curve of the anti-stall has a small inclination and the left side portion Z relative to the flexion point X in the pressure characteristics curve of the anti-stall has a large inclination. Yet, in the case of the one shown inFIG. 6, as compared with the ones shown inFIGS. 8 and 9, the flexion point X is located at a position of the rotational speed of theengine29 being higher (near 1800 rpm).

Therefore, in the case of the one shown inFIG. 6, when the rotational speed of theengine29 drops so that the rotational speed of theengine29 is lower than the flexion point X, there occurs a large drop in the pilot pressure relative to the drop in the rotational speed of theengine29, so that the pilot pressure outputted from the travelingoperating apparatus14 in response to the drop in the rotational speed of theengine29 will respond sensitively and quickly, so that the swash plate of theHST pump53 will be returned quickly so as to decrease the rotational speed of thisHST pump53.

Further,FIG. 7 shows the pressure characteristics curve of the anti-stall in the hydraulic system depicted inFIG. 1.

In the case of the one shown inFIG. 7, the right side portion Y relative to the flexion point X in the pressure characteristics curve of the anti-stall represents the condition when thecharge relief valve56 is opened (the condition when the oil is escaped from thefirst bleed circuit69, theflushing throttle67 and the charge relief valve56), and the left side portion Z relative to the flexion point X in the pressure characteristics curve of the anti-stall represents the condition when thecharge relief valve56 is closed. In the left side portion Z, a right side portion Zb relative to a center portion Za represents the condition when the flushingrelief valve51 is opened so that the oil is escaped from thefirst bleed circuit69 and theflushing throttle67. The center portion Za represents the condition when the flushingrelief valve51 is opened or closed so as to maintain a substantially constant pilot pressure. And, a left side portion Zc relative to the center portion Za represents the condition when the flushingrelief valve51 is completely closed, so that the oil is escaped only from thefirst bleed circuit69.

In the case of the one shown inFIG. 7 too, likeFIG. 6, the flexion point X is located at the position of high rotational speed of theengine29. So, when the rotational speed of theengine29 drops below the flexion point X, there occurs a large pressure drop in the pilot pressure in response to this drop in the rotational speed of theengine29, and the pilot pressure outputted from the travelingoperating apparatus14 in association with the drop in the rotational speed of theengine29 will respond sensitively, so that the swash plate of theHST pump53 will be returned quickly so as to reduce the rotational speed of thisHST pump53. Further, in the case of the one shown inFIG. 7, arrangement is provided such that a substantially constant pressure (10 kgf/cm2) is maintained in the vicinity of the rotational speed of theengine29 ranging from 800 rpm to 1200 rpm. This is provided because a certain level of load may be sometimes applied during the idling rotation of theengine29 also.

For instance, in the case of straight traveling at the time of idling rotation, the swash plate of theHST pump53 can be controlled even with a pilot pressure below 10 kgf/cm2. However, in the case of making a turn at the time of idling rotation, a certain level of load is applied, so that the swash plate of theHST pump53 cannot be controlled unless the pilot pressure is at least 10 kgf/cm2 approximately.

Further, in the present embodiment, there is provided asecond bleed circuit70 for draining, through athrottle70b, a portion of the pilot oil from the second pump P2 in order to switch over theHST motor47 from the first speed condition to the second speed condition.

Thissecond bleed circuit70 includes a bleed oil passage70ahaving one end thereof connected between thesecond switching valve45 and thecylinder switching valve68 of the second supplying passageway (k) and the other end thereof open to thework oil tank31; and athrottle70bincorporated in this bleed oil passageway70a.

When theHST motor47 is under the second speed condition and a load is applied to theengine29, stall of theengine29 tends to occur. So, providing thefirst bleed circuit69 alone may not be sufficient to prevent this. However, with the provision of thesecond bleed circuit70, when theHST motor47 is under the second speed condition and thework machine1 is advanced to plunge thebucket23 into piled sand/earth, due to the leak of the pilot oil from thesecond bleed circuit70 in addition to the leak of the pilot oil from thefirst bleed circuit69, the pilot pressure outputted from the travelingoperating apparatus14 is dropped quickly, thus preventing stall of theengine29.

The implementoperating apparatus15 includes an arm elevatingpilot valve57, an arm loweringpilot valve58, a bucket dumping pilot valve59, a bucket scoopingpilot valve60 and an operating lever (single lever)61 common to thesepilot valves57,58,59,60.

To eachpilot valve57,58,59,60 of the implementoperating apparatus15, pilot oil of the fifth supplying passageway (g) (from the second pump P2) can be supplied, in response to magnetic excitation of an implementlock valve62 comprising an electromagnetic two-position switching valve; and in response to demagnetization of the implementlock valve62, supplying of the pressure oil from the second pump P2 becomes disabled, so that the implementoperating apparatus15 become inoperable.

The implementlock valve62, like the travelinglock valve46 and thebrake releasing valve52, receives a demagnetizing signal by the lock lever operable at the time of driver's getting of the machine and receives a magnetizing signal by a release switch.

The implementcontrol valve33 includes anarm control valve63 for controlling thelift cylinder26, abucket control valve64 for controlling thetilt cylinder28, and an auxiliary control valve65 (this will be referred to as “SP control valve”) for controlling a hydraulic actuator of the attachment attached to e.g. the leading end of thearm22. Each one of thecontrol valves63,64,65 is comprised of a pilot operation type straight moving spool-shaped, three-position switching valve.

Thearm control valve63, thebucket control valve64 and theSP control valve65 are incorporated in an implement supplying oil passageway (r) connected to a discharge passageway (q) of the first pump P1, in the order of thearm control valve63, thebucket control valve64 and theSP control valve65 from the upstream side. In operation, the work oil from the first pump P1 can be supplied to thelift cylinder26 via thearm control valve63, or to thetilt cylinder28 via thebucket control valve64, or to the hydraulic actuator of the attachment via theSP control valve65.

The implement supplying passageway (r) extends through theSP control valve65 and then is connected to the drain oil passageway (s).

In the implement supplying passageway (r), on the upstream side of thearm control valve63, one end of a bypass oil passageway (t) is connected. And, the other end of this bypass oil passageway (t) is connected to the implement supplying oil passageway (r) more downstream than theSP control valve65 and the bypass oil passageway (t) incorporates arelief valve66 for setting the circuit pressure of this implement supplying oil passageway (r).

The operatinglever61 of the implementoperating apparatus15 is operable pivotally with inclination from the neutral position in the forward/reverse and right/left directions and along the oblique directions between the forward/reverse and right/left directions. As the operatinglever61 is pivotally operated, eachpilot valve57,58,59,60 of the implementoperating apparatus15 is operated and also a pilot pressure in proportion to the operational amount of the operatinglever61 from the neutral position is outputted from the operatedpilot valve57,58,59,60.

Further, when the operatinglever61 is pivoted rearward (the direction of arrow B1 inFIG. 3), the arm elevatingpilot valve57 is operated and a pilot pressure is outputted from this arm elevatingpilot valve57. This pilot pressure is applied to one pressure receiving portion of thearm control valve63, whereby thiscontrol valve63 is operated, and thelift cylinder26 is expanded and thearm22 is elevated at the speed proportional to the pivoted amount of the operatinglever61.

When As the operatinglever61 is pivoted forward (the direction of arrow B2 inFIG. 3), the arm loweringpilot valve58 is operated and a pilot pressure is outputted from this arm loweringpilot valve58. This pilot pressure is applied to the other pressure receiving portion of thearm control valve63, whereby thiscontrol valve63 is operated, and thelift cylinder26 is contracted and thearm22 is lowered at the speed proportional to the pivoted amount of the operatinglever61.

When the operatinglever61 is pivoted to the right (the direction of arrow B3 inFIG. 3), the bucket dumping pilot valve59 is operated and a pilot pressure is outputted from this pilot valve59. This pilot pressure is applied to the dumping sidepressure receiving portion64aof thebuck control valve64, whereby thiscontrol valve64 is operated, and thetilt cylinder28 is expanded and thebucket23 effects a dumping action at the speed proportional to the pivoted amount of the operatinglever61.

When the operatinglever61 is pivoted to the left (the direction of arrow B4 inFIG. 3), the bucket scoopingpilot valve60 is operated and a pilot pressure is outputted from thispilot valve60. This pilot pressure is applied to the scooping side pressure receiving portion64bof thebuck control valve64, whereby thiscontrol valve64 is operated, and thetilt cylinder28 is contracted and thebucket23 effects a scooping action at the speed proportional to the pivoted amount of the operatinglever61.

Further, when the operatinglever61 is pivoted in the oblique direction, composite actions of the elevating/lowering action of thearm22 and the scooping/dumping action is made possible.

Further, in the hydraulic system of this embodiment, there is provided a third bleed circuit71 for draining, via athrottle71b, a portion of the pilot oil from the second pump P2 supplied to the scooping side pressure receiving portion64bof thepilot control valve61.

This third bleed circuit71 includes ableed oil passageway71ahaving one end thereof connected to a scooping side pilot oil passageway (u) connecting the bucket scoopingpilot valve60 and the scooping side pressure receiving portion64bof thebucket control valve64, and the other end thereof open to thework oil tank31; and athrottle71bincorporated in thisbleed oil passageway71a.

When thework machine1 is advanced to plunge thebucket23 into piled sand/earth and thebucket23 is caused to effect a scooping action, providing thefirst bleed circuit69 alone may be insufficient to cope with this. However, with further provision of the third bleed circuit71, when thework machine1 is advanced to plunge thebucket23 into piled sand/earth and thebucket23 is caused to effect a scooping action, due to the leak of pilot oil from the third bleed circuit71 in addition to the leak of pilot oil from thefirst bleed circuit69, the pilot pressure outputted from the travelingoperating apparatus14 will drop quickly, thus preventing stall of theengine29.

Second Embodiment

Next, with reference toFIGS. 10-15, a second embodiment will be described. The hydraulic system relating to this embodiment is also applicable to a truck loader (work machine)1 shown inFIGS. 4-5.

In the hydraulic system of thework machine1 shown inFIGS. 10-14, the first through third pumps P1, P2, P3 each is comprised of a fixed displacement type gear pump driven by the power of theengine29.

The first pump P1 (main pump) is used for driving a hydraulic actuator of an attachment attached to the leading end of thelift cylinder26, thebucket cylinder28 or thearm22.

The second pump P2 (pilot pump or charge pump) is used mainly for supplying a control signal pressure (pilot pressure).

The third pump P3 (sub pump) is used for increasing a flow rate of work oil supplied to a hydraulic actuator of a hydraulically driven attachment attached to the leading end of thearm22 in case this hydraulic actuator requires a large capacity.

The travelingoperating apparatus14 includes a forward travelingpilot valve136, a reverse travelingpilot valve137, a right turningpilot valve138, a leftturning pilot valve139, a (single) travelinglever140 common to thesepilot valves136,137,138,139 and first throughfourth shuttle valves141,142,143,144.

The travelingoperating apparatus14 includes first throughfourth output ports146,147,148,149, apump port150 for receiving oil from the second pump P2, and atank port151 communicated with thework oil tank31.

To the second pump P2, there is connected a main supply passageway (a1) for flowing the discharge oil (pilot oil) discharged from this second pump P2. To this main supplying passageway (a), there is connected arelief circuit153 including arelief valve152.

The main supply passageway (a1) is branched at a branching point (b1) into a first supplying passageway (c1), a second supplying passageway (d1) and a third supplying passageway (e1), and the third supplying passageway (e1) is further branched into a fourth supplying passageway (f1) and a fifth supplying passageway (g1).

The first supplying passageway (c1) is connected to apump port150 of the travelingoperating apparatus14, so that the discharge oil from the second pump P2 is supplied as pilot oil to the travelingoperating apparatus14 and this pilot oil supplied to the travelingoperating apparatus14 can be supplied to each of thepilot valves136,137,138,139 of the travelingoperating apparatus14, and unused pilot oil is drained from atank port151.

The first supplying passageway (c1) includes a travelinglock valve154 comprising an electromagnetic type two-position switch valve and apressure compensating valve155 located between this travelinglock valve154 and the branching point (b1).

The pilot oil flowing in the first supplying passageway (c1) can be supplied to the travelingoperating apparatus14 with magnetic excitation of the travelinglock valve154. When this travelinglock valve154 is demagnetized, the pilot oil of the first supplying oil passageway (c) cannot be supplied to the travelingoperating apparatus14, so that the travelingoperating apparatus14 becomes inoperable.

Thepressure compensating valve155 maintains the primary pressure of thispressure compensating valve155 and the pressures of the second through fifth supplying passageways (d1-g1) at predetermined pressures (e.g. 30 kgf/cm2).

The hydraulic system of thiswork machine1 includes a bleed circuit156 (this will be referred to as the “main bleed circuit”) for draining a portion of the discharge oil from the second pump P2 through athrottle156a.

Thismain bleed circuit156 is provided for draining a portion of the pressure oil on the upstream side of thepressure compensating valve155, and includes ableed oil passageway156bhaving one end thereof connected to the oil passageway upstream of thepressure compensating valve155 and therelief valve152 and the other end thereof communicated to thework oil tank31, and thethrottle156aincorporated in thisbleed oil passageway156b.

The diameter of thethrottle156aof thismain bleed circuit156 is sized sufficient to maintain the primary pressure of thepressure compensating valve155 and the pressures of the second through fifth supplying passageways (d1-g1) at predetermined pressures (e.g. 30 kgf/cm2), even when theengine29 is under idling rotation (about 1050 rpm), and sized also to allow the primary pressure of thispressure compensating valve155 and the pressures of the second through fifth supplying passageways (d1-g1) to drop to an approximately half of the set pressure of the pressure compensating valve155 (e.g. about 15 kgf/cm2).

Each of the right/left traveling motors21L,21R includes anHST motor157 comprising a swash plate, variable displacement type axial motor switchable in two, high/low speeds, a swashplate switching cylinder158 for speed-changing theHST motor157 between the two, high/low speeds by switching over the angle of the swash plate, abrake cylinder159 for braking anoutput shaft157aof the HST motor157 (output shaft157aeach of the travelingmotors21L,21R), a flushingvalve160, aflushing relief valve161 and aflushing throttle162.

The swashplate switching cylinder158, when no pressure oil is being applied thereto, renders theHST motor157 into the first speed condition and renders, when pressure oil is being applied thereto, theHST motor157 into the second speed condition.

Whether to apply the pressure oil to this swashplate switching cylinder158 or not is determined by thecylinder switching valve163 comprising a pilot operation type two-position switching valve and thiscylinder switching valve163 can be switched over by a two-speed switching valve164 comprising an electromagnetic type, two-position switching valve.

In more particular, when the two-speed switching valve164 is demagnetized so that the second supplying passageway (d1) is shut by this two-speed switching valve164, no pilot pressure is applied to thecylinder switching valve163 and also no pressure oil is applied to the swashplate switching cylinder158, so that theHST motor157 is under the first speed condition. And, when the two-speed switching valve164 is excited by operating means, the two-speed switching valve164 is switched over so as to apply the pilot pressure of the second supplying passageway (d1) (the discharge oil from the second pump P2) to thecylinder switching valve163, whereby thecylinder switching valve163 is switched over to apply the pressure oil to the swashplate switching cylinder158, so that theHST motor157 is rendered into the second speed condition.

Thebrake cylinder159 brakes theoutput shaft157aof theHST motor157 with an urging force of a spring, and as abrake release valve165 comprising an electromagnetic type two-position switching valve is magnetized, the pilot oil of the fourth supplying passageway (f1) (the discharge oil from the second pump P2) is applied to thisbrake cylinder159, thus releasing the braking of theoutput shaft157aof theHST motor157.

To the travelinglock valve154 and thebrake release valve165, demagnetizing signals are transmitted simultaneously by means of e.g. a lock lever which is operated when a driver gets out of thecabin5 and magnetizing signals are transmitted simultaneously thereto by a release switch.

The flushingvalve160 and theflushing relief valve161 will be described later herein.

Thehydraulic drive apparatus32 includes a left travelingmotor driving circuit32A (left driving circuit) for theleft traveling motor21L and a right travelingmotor driving circuit32B (right driving circuit) for theright traveling motor21R. Each drivingcircuit32A,32B includes an HST pump (traveling hydraulic pump)166 connected to theHST motor157 of the travelingmotor21R or21L corresponding thereto via a pair of speed-changing oil passageways (h1, i1); a highpressure relief valve167 for releasing the pressure to the low pressure side of the speed-changing oil passageways (h1, i1) when the pressure of the high pressure side of the speed-changing oil passageways (h1, i1) exceeds a predetermined pressure; and a charge circuit (j1) for supplementing pressure oil from the second pump P2 to the low pressure side oil passageway (h1, i1) via thecheck valve168.

The components of thehydraulic drive apparatus32 are incorporated within a housing.

Thehydraulic drive apparatus32 includes first through fourth input ports169-172 for inputting the pilot oil from the travelingoperating apparatus14.

Thefirst input port169 is connected via a firstoutput oil passageway173 to thefirst output port146 of the travelingoperating apparatus14. Thesecond input port170 is connected via a secondoutput oil passageway174 to thesecond output port147 of the travelingoperating apparatus14. Thethird input port171 is connected via a thirdoutput oil passageway175 to thethird output port148 of the travelingoperating apparatus14. Thefourth output port172 is connected via a fourthoutput oil passageway176 to thefourth output port149 of the travelingoperating apparatus14.

Each one of the first through fourth output oil passageways173-156 includes ashock relieving throttle177.

The charge circuit (j1) can receive the oil of the charge pressure supplying passageway (k1) (discharge oil from the second pump P2 and the primary pilot oil of the traveling operating apparatus14) branched from the first supplying passageway (c1) and connected to each charge circuit61).

The charge pressure supplying passageway (k1) is branched from the first supplying passageway (c) on the downstream side of thepressure compensating valve155 and on the upstream side of the travelinglock valve154, and is connected to the charge circuit (j1).

Further, theleft driving circuit32A incorporates acharge relief valve178 for setting the pressure of the charge circuit (j1) each of the drivingcircuits32A,32B.

The second pump P2, in this embodiment, functions not only as a pilot pump for supplying pilot oil to thepilot valves136,137,138,139 of the travelingoperating apparatus14 and to thecylinder switching cylinder163 and thebrake cylinder159, but also as a charge pump for supplying oil to the charge circuit (j1).

TheHST pump166 each of the drivingcircuits32A,32B functions not only as a swash plate variable displacement type axial pup driven by the power of theengine29, but also as a pilot type hydraulic pump having a swash plate whose angle is changed by the pilot pressure (swash plate, variable displacement hydraulic pump).

In more particular, theHST pump166 includes a forward travelingpressure receiving portion166aand a reverse travelingpressure receiving portion166bto which the pilot pressure is applied. As the swash plate angle is changed by the pilot pressure applied to thesepressure receiving portions166a,166b, a direction and an amount of work oil discharged from theHST pump166 are changed, whereby the rotational output each of the travelingmotors21L,21R can be speed-changed in stepless manner in the direction for moving thework machine1 forwardly (forward rotation direction) or in the direction for moving thework machine1 in reverse (reverse rotation direction).

The forward travelingpressure receiving portion166aof the HST pump166 of theleft driving circuit32A is connected to thefirst input port169 via a first connectingoil passageway179, and the reverse travelingpressure receiving portion166bof theHST pump166 is connected to thethird input port171 via a second connectingoil passageway180.

The forward travelingpressure receiving portion166aof the HST pump166 of theright driving circuit32B is connected to thefourth input port172 via a third connectingoil passageway181 and the reverse travelingpressure receiving portion166bof theHST pump166 is connected to thesecond input port170 via a fourth connectingoil passageway182.

The first connectingoil passageway179 and the third connectingoil passageway181 are communicated to the charge pressure supplying passageway (k1) (the primary side oil passageway of the traveling operating apparatus14) via anescape oil passageway182.

Thisescape oil passageway183 has its one end connected to the charge pressure supplying passageway (k1).

Further, the other end of theescape oil passageway183 is branched into afirst branch passageway183aand asecond branch passageway183b, and thefirst branch passageway183ais connected to the first connectingoil passageway179 and thesecond branch passageway183bis connected to the third connectingoil passageway181. And, the other end of theescape oil passageway183 is communicated to the forward travelingpressure receiving portion166aof theHST pump166.

Thefirst branch passageway183aand thesecond branch passageway183beach incorporates acheck valve184.

Thischeck valve184 is closed when the primary pressure of the travelingoperating apparatus14 is greater than the pressure of the forward travelingpressure receiving portion166a, thus preventing communication of pressure oil from the charge pressure supplying passageway (k1) to the first and third connectingoil passageways179,181. And, thisvalve184 is opened when the primary pressure of the travelingoperating apparatus14 is smaller than the pressure of the forward travelingpressure receiving portion166a, thus allowing communication of pressure oil from the first and third connectingoil passageways179,181 to the charge pressure supplying passageway (k1).

Incidentally, the other end of theescape oil passageway183 may be connected to the hydraulic passageway between theshock relieving throttle177 and the forward travelingpressure receiving portion166a.

The flushingvalve160 each of the travelingmotors21L,21R is switched over by the pressure of the high pressure side of the speed-changing oil passageways (h1, i1) to connect the low pressure side of the speed-changing oil passageways (h1, i1) to the flushing relief oil passageway (m1), and flushes a portion of the work oil of the low speed side oil passageway (h1, i1) via a flushing relief oil passageway (m1) to the oil pan inside the housing each of the travelingmotors21L,21R so as to supplement an amount of work oil to the low pressure side of the speed-changing oil passageways (h1, i1). Incidentally, the oil in the oil pan inside the housing each of the travelingmotors21L,21R is returned to thework oil tank31 via a drain circuit (n1).

The flushingrelief valve161 and theflushing throttle162 are incorporated in the flushing relief oil passageway (m1) and theflushing relief valve161 is interposed between the flushingvalve160 and theflushing throttle162.

TheHST motor157 and theflushing valve160, etc. each of the travelingmotors21L,21R, the drivingcircuits32A,32B and the pair of speed-changing oil passageways (h1, i1) together constitute a separate type HST (hydrostatic transmission).

A travelinglever140 of the travelingoperating apparatus14 is operable pivotally with inclination from the neutral position in the forward/reverse and right/left directions and along the oblique directions between the forward/reverse and right/left directions. As the travelinglever140 is pivotally operated, eachpilot valve136,137,138,139 of the travelingoperating apparatus14 is operated and a pilot pressure proportional to the operated amount of the travelinglever140 from the neutral position is outputted from the operatedpilot valve136,137,138,139.

As the travelinglever140 is operated to the front side (the arrow direction A1 inFIG. 11), the forward travelingpilot valve136 is operated so that pilot pressure is outputted from thispilot valve136, and this pilot pressure is applied to the forward travelingpressure receiving portion166aof the HST pump166 of theleft driving circuit32A via thefirst shuttle valve141 and applied also to the forward travelingpressure receiving portion166aof theright driving circuit32B via thesecond shuttle valve142, whereby theoutput shafts157aof the right/left traveling motors21L,21R are driven forwardly (forward rotation) at a speed in proportion to the pivotal amount of the travelinglever140, so that thework machine1 travels forward.

Further, as the travelinglever140 is operated to the rear side (the arrow direction A2 inFIG. 11), the reverse travelingpilot valve137 is operated so that pilot pressure is outputted from thispilot valve137, and this pilot pressure is applied to the reverse travelingpressure receiving portion166bof the HST pump166 of theleft driving circuit32A via thethird shuttle valve143 and applied also to the reverse travelingpressure receiving portion166bof the HST pump166 of theright driving circuit32B via thefourth shuttle valve144, whereby theoutput shafts157aof the right/left traveling motors21L,21R are driven reversely (reverse rotation) at a speed in proportion to the pivotal amount of the travelinglever140, so that thework machine1 travels reverse.

Further, as the travelinglever140 is operated to the right side (the arrow direction A3 inFIG. 11), the right turningpilot valve138 is operated so that pilot pressure is outputted from thispilot valve138, and this pilot pressure is applied to the forward travelingpressure receiving portion166aof the HST pump166 of theleft driving circuit32A via thefirst shuttle valve141 and applied also to the reverse travelingpressure receiving portion166bof the HST pump166 of theright driving circuit32B via thefourth shuttle valve144, whereby theoutput shaft157aof theleft driving motor21L is rotated forwardly and also theoutput shaft157aof theright traveling motor21R is rotated in reverse, so that thework machine1 turns to the right side.

Further, as the travelinglever140 is operated to the left side (the arrow direction A4 inFIG. 11), the left turningpilot valve139 is operated so that pilot pressure is outputted from thispilot valve139, and this pilot pressure is applied to the forward travelingpressure receiving portion166aof the HST pump166 of theright driving circuit32B via thesecond shuttle valve142 and applied also to the reverse travelingpressure receiving portion166bof the HST pump166 of theleft driving circuit32A via thethird shuttle valve143, whereby theoutput shaft157aof theright driving motor21R is rotated forwardly and also theoutput shaft157aof theleft traveling motor21L is rotated in reverse, so that thework machine1 turns to the left side.

Still further, as the travelinglever140 is pivoted along the oblique direction, due to the pressure difference between the pilot pressures applied to the forward travelingpressure receiving portion166aand the reverse travelingpressure receiving portion166beach of the drivingcircuits32A,32B, the rotational direction and the rotational speed of theoutput shaft157aeach of the travelingmotors21L,21R are determined, such that thework machine1 will make a right turn or a left turn while traveling forwardly or in reverse. (More particularly, when the travelinglever140 is pivoted in the forward obliquely left direction, thework machine1 will turn left while traveling forward at the speed corresponding to the pivotal angle of the travelinglever140. When the travelinglever140 is pivoted in the forward obliquely right direction, thework machine1 will turn right while traveling forward at the speed corresponding to the pivotal angle of the travelinglever140. When the travelinglever140 is pivoted in the reverse obliquely left direction, thework machine1 will turn left while traveling reverse at the speed corresponding to the pivotal angle of the travelinglever140. When the travelinglever140 is pivoted in the reverse obliquely right direction, thework machine1 will turn right while traveling reverse at the speed corresponding to the pivotal angle of the travelinglever140.)

The supplying of the pilot oil from the travelingoperating apparatus14 to the forward travelingpressure receiving portion166aand the reverse travelingpressure receiving portion166bof theHST pump166 and the returning of the pilot oil from the forward travelingpressure receiving portion166aand the reverse travelingpressure receiving portion166bare effected via theshock relieving throttle177, so sudden changes in the vehicle speed can be avoided.

Further, theengine29 can be accelerated by an accelerator from the idling rotational speed to a rated rotational speed. When the rotational speed of theengine29 is increased, the rotational speed of theHST pump166 is increased, whereby the discharge amount of this HST pump166 is raised, and the traveling speed is increased.

To the charge pressure supplying passageway (k1), there is connected a bleed circuit185 (this will be referred to as “a travelingbleed circuit185” hereinafter).

This travelingbleed circuit185 includes ableed oil passageway185ahaving one end thereof connected to the charge pressure supplying passageway (k1) and the other end thereof communicated to the oil pan of the housing of thehydraulic drive apparatus32 and further includes athrottle185bincorporated in thisbleed oil passageway185a.

The oil of the oil pan of the housing of thehydraulic drive apparatus32 is returned to thework oil tank31 via the drain circuit (n1).

The pilot oil discharged from the second pump P2 and supplied to the travelingoperating apparatus14 via the first supplying passageway (c1) is supplied also to the charge circuit (j1) via the charge pressure supplying passageway (k1) and a portion thereof is drained by the travelingbleed circuit185 through thethrottle185bof thisbleed circuit185.

Incidentally, the oil drained via the travelingbleed circuit185 could be directly returned to thework oil tank31. However, as this oil is drained to the inside of the housing of the hydraulic drive apparatus32 (i.e. the housing of the HST pump166), cooling of theHST pump166, etc. is made possible.

Further, the flushingrelief valve161 can be omitted in the hydraulic system described above.

Still further, the other end of thebleed oil passageway185acan be communicated to the relief oil passageway (o1) for guiding the oil to be drained from thecharge relief valve178 to the oil pan of the housing of thehydraulic drive apparatus32,

With thework machine1 having the above-described arrangements, when e.g. thework machine1 is advanced to plunge thebucket23 into piled-up sand/earth or the like, a load will be applied to theHST motor157. In this, this load applied to theHST motor157 will be transmitted via theHST pump166 to theengine29, whereby the rotational speed of theengine29 will drop.

Then, the rotational speed of the second pump P2 will decrease, thus decreasing the discharge amount of this second pump P2 and the ratio of the oil leak from the travelingbleed circuit185 relative to this discharge amount of the second pump P2 will become larger. As a result, the primary pressure of the travelingoperating apparatus14 will drop and the pilot pressure outputted from the travelingoperating apparatus14 will drop speedily according to the reduction in the rotational speed of theengine29. With this, the swash plate angle of theHST pump166 will be automatically adjusted in a speedy manner so as to reduce the rotational speed (i.e. so as to return the swash plate toward its neutral position), thus reducing the load applied to theengine29. As a result, the stall of theengine29 can be avoided effectively.

Even if the travelingbleed circuit185 were not provided, due to the escape of the oil from theflushing throttle162 and the override characteristics of the charge relief valve178 (the escape of oil from the charge relief valve178), there could be provided the anti-stall characteristics for reducing the control pressure of the swash plate of theHST pump166 in response to reduction in the rotational speed of theengine29. But, the anti-stall effect of such arrangement would be small.

In contrast, according to the present embodiment, in addition to the escape of the oil from theflushing throttle162 and the override characteristics of thecharge relief valve178, thanks to the leak of oil from the travelingbleed circuit185, effective anti-stall characteristics (effective anti-stall performance) can be obtained.

In the case of the arrangement wherein the supplying and discharging of the pilot oil to/from thepressure receiving portions166a,166bof theHST pump166 in order to avoid sudden change in the vehicle speed, when thework machine1 is advanced to plunge thebucket23 into piled sand/earth and an excessive load is applied suddenly to theengine29, if pressure relief of the forward travelingpressure receiving portion166ais effected via theshock relieving throttle177, the pressure drop of the forward travelingpressure receiving portion166aof theHST pump166 may lag behind the drop of the primary pressure of the travelingoperating apparatus14.

However, in the present embodiment, when the primary pressure of the travelingoperating apparatus14 becomes smaller than the pressure of the forward travelingpressure receiving portion166a, thecheck valve184 provided in theescape oil passageway183 is opened, thus allowing communication of pressure oil from the first and third connectingoil passageways179,181 to the charge pressure supplying passageway (k1), and thus relieving the pressure of the forward travelingpressure receiving portion166a. Therefore, even when an excessive load is applied suddenly to theengine29, the swash plate of theHST pump166 will be returned quickly to the neutral side, so that stall of theengine29 can be prevented.

Accordingly, in the present invention, the response of the anti-stall feature can be improved with the simple arrangement of providing a check valve in theescape oil passageway183, while sudden speed change is prevented by theshock relieving throttle177.

Incidentally, in the present embodiment, the escape circuit comprising theescape oil passageway183 and thecheck valve184 is provided for the forward travelingpressure receiving portion166a. Instead, this circuit may be provided for the reverse travelingpressure receiving portion166b.

Further, in thework machine1 having the above-described construction, when the rotational speed of theengine29 drops and the discharge amount of the second pump P2 is decreased and due to the anti-stall function by the travelingbleed circuit185, etc., the primary pressure of the travelingoperating apparatus14 becomes smaller than the set pressure of thepressure compensating valve155, and thus thepressure compensating valve155 is closed. However, in the case of the arrangement having nomain bleed circuit156, if thepressure compensating valve155 is closed in such case as above, there will be no place for allowing escape of the discharge oil from the second pump P2. So, the pressure on the upstream side of thepressure compensating valve155 becomes high, so thepressure compensating valve155 will be opened again. And, when the primary pressure of the travelingoperating apparatus14 becomes lower than the set pressure of thepressure compensating valve155, thepressure compensating valve155 will be closed. So, these opening/closing operations will be repeated.

In contrast to the above, in the case of the arrangement of the present embodiment wherein themain bleed circuit156 is provided for draining a portion of the pressure oil on the upstream side of thepressure compensating valve155, when the rotational speed of theengine29 drops and the discharge flow rate of the second pump P2 is decreased, thus closing thepressure compensating valve155, thereafter, due to the leak of the oil from themain bleed circuit156, the pressure on the upstream side of thepressure compensating valve155 will not rise, so that thepressure compensating valve155 will remain closed and the pressure oil from the second pump P2 will not flow to the downstream side of the pressure compensating valve155 (traveling bleed circuit185).

Therefore, with the provision of themain bleed circuit156, it is possible to allow the anti-stall function by the travelingbleed circuit185 to manifest itself effectively.

The implementoperating apparatus15 includes an arm elevatingpilot valve186, an arm loweringpilot valve187, a bucket dumpingpilot valve188, a bucket scoopingpilot valve189 and an operating lever (single lever)190 common to thesepilot valves186,187,188,189.

To eachpilot valve186,187,188,189 of the implementoperating apparatus15, pilot oil from the second pump P2 can be supplied via the fifth supplying passageway (g1), in response to magnetic excitation of an implementlock valve191 comprising an electromagnetic two-position switching valve; and in response to demagnetization of the implementlock valve191, supplying of the pressure oil form the second pump P2 becomes disabled, so that the implementoperating apparatus15 becomes inoperable.

The implementlock valve191, like the travelinglock valve154 and thebrake releasing valve165, receives a demagnetizing signal by the lock lever operable at the time of driver's getting out of thecabin5 and receives a magnetizing signal by a release switch.

The implementcontrol valve33 includes anarm control valve192 for controlling thelift cylinder26, abucket control valve193 for controlling thebucket cylinder28, and an auxiliary control valve194 (this will be referred to as “SP control valve”) for controlling a hydraulic actuator of the attachment attached to e.g. the leading end of thearm22. Each one of thecontrol valves192,193,194 is comprised of a pilot operation type straight moving spool-shaped, three-position switching valve.

Thearm control valve192, thebucket control valve193 and theSP control valve194 are incorporated in an implement supplying oil passageway (r1) connected to a discharge passageway (q1) of the first pump P1, in the order of thearm control valve192, thebucket control valve193 and theSP control valve194 from the upstream side, thereby to constitute a serial circuit. In operation, the work oil from the first pump P1 can be supplied to thelift cylinder26 via thearm control valve192, or to thebucket cylinder28 via thebucket control valve193, or to the hydraulic actuator of the attachment via theSP control valve194.

The implement supplying passageway (r1) extends through theSP control valve194 and is then connected to the drain oil passageway (s1).

In the implement supplying passageway (r1), on the upstream side of thearm control valve192, one end of a bypass oil passageway (t1) is connected. And, the other end of this bypass oil passageway (t1) is connected to the implement supplying oil passageway (r1) more downstream than theSP control valve194 and the bypass oil passageway (t1) incorporates arelief valve196 for setting the circuit pressure of this implement supplying oil passageway (r1).

The implementcontrol valve33 further includes anarm relief valve197 for protecting thelift cylinder26, abucket relief valve198 for protecting thebucket cylinder28, and anSP relief valve199 for protecting a hydraulic actuator of the attachment.

An operatinglever190 of the implementoperating apparatus15 is operable pivotally with inclination from the neutral position in the forward/reverse and right/left directions and along the oblique directions between the forward/reverse and right/left directions. As the operatinglever190 is pivotally operated, eachpilot valve186,187,188,189 of the implementoperating apparatus15 is operated and also a pilot pressure in proportion to the operational amount of the operatinglever190 from the neutral position is outputted from the operatedpilot valve186,187,188,189.

Further, when the operatinglever190 is pivoted rearward (the direction of arrow B1 inFIG. 13), the arm elevatingpilot valve186 is operated and a pilot pressure is outputted from this arm elevatingpilot valve186. This pilot pressure is applied to one pressure receiving portion of thearm control valve192, whereby thiscontrol valve192 is operated, and thelift cylinder26 is expanded and thearm22 is elevated at the speed proportional to the pivoted amount of the operatinglever190.

When the operatinglever190 is pivoted forward (the direction of arrow B2 inFIG. 13), the arm loweringpilot valve187 is operated and a pilot pressure is outputted from this arm loweringpilot valve187. This pilot pressure is applied to the other pressure receiving portion of thearm control valve192, whereby thiscontrol valve192 is operated, and thelift cylinder26 is contracted and thearm22 is lowered at the speed proportional to the pivoted amount of the operatinglever190.

When the operatinglever190 is pivoted to the right (the direction of arrow B3 inFIG. 13), the bucket dumpingpilot valve188 is operated and a pilot pressure is outputted from thispilot valve188. This pilot pressure is applied to the dumping sidepressure receiving portion193aof thebucket control valve193, whereby thiscontrol valve193 is operated, and thebucket cylinder28 is expanded and thebucket23 effects a dumping action (downward pivotal action) at the speed proportional to the pivoted amount of the operatinglever190.

When the operatinglever190 is pivoted to the left (the direction of arrow B4 inFIG. 13), the bucket scoopingpilot valve189 is operated and a pilot pressure is outputted from thispilot valve189. This pilot pressure is applied to the scooping sidepressure receiving portion193bof thebuck control valve193, whereby thiscontrol valve193 is operated, and thebucket cylinder28 is contracted and thebucket23 effects a scooping action (upward pivotal action or a raking-in action) at the speed proportional to the pivoted amount of the operatinglever190.

Further, when the operatinglever190 is pivoted in the oblique direction, composite actions of the elevating/lowering action of thearm22 and the scooping/dumping action is made possible.

Thebucket control valve193 comprises a five-port, three-position switching valve including first andsecond pump ports201,202 for receiving pressure oil from the first pump P1, a cylinder port203 (this will be referred to as “a scooping port”) communicated to the rodside oil chamber28aof thebucket cylinder28, a cylinder port204 (this will be referred to as “a dumping port”) communicated to the bottomside oil chamber28bof thebucket cylinder28, and atank port205 communicated to the drain oil passageway (s1) via theSP control valve194; and switchable into aneutral position193A for not operating thebucket23, ascooping position193B for causing thebucket23 to effect a scooping action and a dumpingposition193C for causing thebucket23 to effect a dumping action.

With thisbucket control valve193 in operation, at theneutral position193A, communication between thefirst pump port201 and thetank port205 is established and also thesecond pump port202, the scoopingport203 and the dumpingport204 are shut. At thescooping position193B, communication is established between thesecond pump port202 and the scoopingport203 is established and also communication is established between thetank port205 and the dumpingport204, whereby pressure oil is supplied to the rodside oil chamber28aof thebucket cylinder28 and also the pressure oil is discharged from the bottom side oil chamber82b. At the dumpingposition193C, communication is established between thesecond pump port202 and the dumpingport204 and also communication is established between thetank port205 and the scoopingport203, whereby the pressure oil is supplied to the bottomside oil chamber28bof thebucket cylinder28 and also the pressure oil is discharged from the rodside oil chamber28a.

Further, thefirst pump port201 is shut at the dumpingposition193C of thebucket control valve193, but at thescooping position193B, thefirst pump port201 is communicated to thetank port205 via a bleed circuit206 (this will be referred to as “a bucket bleed circuit”), and at thisscooping position193B, a portion of the pressure oil supplied from the first pump P1 to thebucket cylinder28 is drained via thisbucket bleed circuit206.

Thisbucket bleed circuit206 includes ableed oil passageway206acommunicating thefirst pump port201 to thetank port205, and athrottle206bincorporated in thisbleed oil passageway206a.

Further, at thescooping position193B of thebucket control valve193, one end of thereturn oil passageway207 for returning the oil discharged from the bottomside oil chamber28bof thebucket cylinder28 to thetank port205 is communicated to the dumpingport204 and the other end of thereturn oil passageway207 is connected to the upstream side of thethrottle206bin thebleed oil passageway206a.

Further, thisreturn oil passageway207 incorporates acheck valve208 for checking communication of pressure oil from thebleed oil passageway206ato the dumpingport204.

With thebucket control valve193 having the above-described construction, when thebucket23 is caused to effect a scooping action, if theengine29 is at a high rotational speed (e.g. the rated rotation: 2400 rpm), the amount of leak from thebucket bleed circuit206 is small relative to the discharge flow amount of the first pump P1 (i.e. there is hardly any drop in the pressure of the pressure oil supplied from the first pump P1 to the bucket cylinder28), a high-load operating for the scooping action of thebucket23 is possible. However, as the rotational speed of theengine29 decreases, the ratio of the leak amount from thebucket bleed circuit206 relative to the discharge flow amount of the first pump P1 progressively increases, so that the pressure drop of the pressure oil supplied from the first pump P1 to thebucket cylinder28 becomes larger, so high-load operation becomes impossible.

With this, the work load of thebucket23 falls automatically in association with the drop in the rotational speed of the engine. Hence, the load to the first pump P1 is alleviated and the consumption torque of theengine29 will decrease.

The leak amount of the bucket bleed circuit206 (the diameter of thethrottle206b) will be set such that e.g. at the time of idling rotation of theengine29, the maximum pressure of the pressure oil supplied from the first pump P1 to thebucket cylinder28 at the time of scooping action may be maintained at a pressure substantially equal to the set pressure of the main relief valve196 (in other words, when the rotational speed of the engine is below the idling rotational speed, themain relief valve196 will not be opened at the time of scooping action).

Further, in the case of the above-described construction, thebucket23 is caused to effect a scooping action by supplying the pressure oil to the rodside oil chamber28aof thebucket cylinder28 for contracting this bucket cylinder28 (retracting the piston rod of the bucket cylinder28). So, the speed of the action of the bucket23 (bucket cylinder28) is higher at the time of the scooping action than the time of dumping action, in case thebucket bleed circuit206 is not provided. Therefore, even if the pressure oil from the first pump P1 is leaked by thebucket bleed circuit206, there occurs no problem in the work speed matching between the scooping action and the dumping action times of thebucket23.

In the case of an arrangement where thebucket bleed circuit206 is not provided, when thework machine1 is advanced to plunge thebucket23 into piled sand/earth and thebucket23 is caused to effect a scooping action and thebucket relief valve198 is opened, the provision of the travelingbleed circuit185 alone may not be sufficient to cope with engine stall. However, by providing thebucket bleed circuit206 in addition to the travelingbleed circuit185, when thework machine1 is advanced and thebucket23 is plunged into pile of sand/earth to effect a scooping action, the load to theengine29 is alleviated by the travelingbleed circuit185 and further the load to the first pump P1 is alleviated by thebucket bleed circuit206, so that the load of theengine29 can be alleviated more. With these combined, the stall of theengine29 in case thework machine1 is advanced and thebucket23 is plunged into pile of sand/earth to effect a scooping action (a situation when high load is applied to theHST pump166 and the first pump P1 simultaneously) can be prevented effectively.

Further, in the case of the arrangement as in the present embodiment wherein thearm control valve192 is provided on the upstream side of thebucket control valve193, when thearm control valve192 effects a full stroke action, the pressure oil from the first pump P1 does not flow to thebucket control valve193, so the above-described effect of thebucket bleed circuit206 is disabled. So, it may not be possible to cope with the engine stall when thework machine1 is advanced to plunge thebucket23 into piled sand/earth and thebucket23 is caused to effect a scooping action and thearm relief valve197 is opened with the spool of thearm control valve192 making a full stroke action.

However, to cope with such a problem as above, in the case of thework machine1 of this embodiment, since themain bleed circuit156 is provided on the upstream side of thepressure compensating valve155 for draining a portion of the pilot oil supplied from the second pump P2 to the implementoperating apparatus15, when the rotational speed of theengine29 drops below the idling rotation, the pressure of the fifth supplying passageway (g1) may drop below the pressure set by thepressure compensating valve155.

With the above-described arrangement, the primary and second pressures of the implementoperating apparatus15 will decrease and the pilot pressure outputted from the implementoperating apparatus15 prevents full stroke actions of the spools of thearm control valve192 and thebucket control valve193.

And, as the result of the spool of thearm control valve192 making no full stroke action, with communication of the pressure oil to thebucket control valve193 from thearm control valve192 via the implement supplying oil passageway (r1), the effect of thebucket bleed circuit206 is allowed to manifest itself, such that rise in the pressure of the pressure oil discharged from the first pump P1 is restricted, thus alleviating the load to theengine29. Consequently, it is possible to effectively prevent occurrence of engine stall in case thework machine1 is advanced and thebucket23 is caused to effect a scooping action and thearm22 is operated.

Further, since themain bleed circuit156 functions to prevent full stroke action of the spool of thearm control valve192 by the pilot pressure outputted from the implementoperating apparatus15 in case the rotational speed of theengine29 drops below the idling rotation, even in such case when thebucket23 is not operated at all, but high load is applied to the traveling apparatuses4 and high load is applied to thearm22 as being operated, a portion of the pressure oil supplied from the first pump P1 to thelift cylinder26 via thearm control valve192 is caused to pass thebucket control valve193 into the drain oil passageway (s1), and the pressure of the pressure oil from the first pump P1 will drop, so that the load to the first pump P1 can be alleviated (the load to theengine29 can be alleviated), so stall of theengine29 can be prevented effectively.

Incidentally, as described hereinbefore, since the diameter of thethrottle156aof themain bleed circuit156 is set such that even at the time of idling rotation of theengine29, the pressure of the fifth supplying passageway (g1) can be maintained at the set pressure of thepressure compensating valve155, it is possible to ensure pressure sufficient to allow the spools of thearm control valve192 and thebucket control valve193 to effect full stroke actions even at the time of idling rotation of theengine29.

Further Embodiment of the Hydraulic System of the Implement Line

In the foregoing embodiment, themain bleed circuit156 constitutes pressure reducing means for reducing the primary pressure of the implementoperating apparatus15 when the rotational speed of theengine29 drops.

On the other hand, in the case of a hydraulic system depicted inFIG. 15, as means for coping with engine stall (as “pressure reducing means”) when thework machine1 is advanced to plunge thebucket23 into piled sand/earth and thebucket23 is caused to effect a scooping action and thearm22 is operated, instead of themain bleed circuit156, the primary pressure of the implementoperating apparatus15 is controlled by apressure reducing valve209.

Thepressure reducing valve209 comprises an electromagnetic proportional pilot operation typepressure reducing valve209, which is incorporated in the fifth supplying passageway (g1).

Moreover, in the hydraulic system of the embodiment, there are provided arotation sensor210 for detecting rotational speed of theengine29 and acontroller211 for controlling thepressure reducing valve209. A rotational speed of theengine29 detected by therotation sensor210 is inputted to thecontroller211.

Further, the implementlock valve191 is not provided, but thepressure reducing valve209 provides the role of this implementlock valve191.

In the case of the hydraulic system shown inFIG. 15, when the rotational speed of theengine29 is higher than the idling rotation speed, thecontroller211 generates an instruction signal for rendering thepressure reducing valve209 into its full open condition (no pressure adjustment), so that the primary pressure of the implementoperating apparatus15 is maintained at the set pressure of thepressure compensating valve155.

And, when the rotational speed of theengine29 drops below the idling rotation speed, thecontroller211 will generate an instruction signal for causing thepressure reducing valve209 to effect pressure adjustment such that the primary pressure of the implementoperating apparatus15 may drop below the set pressure of the pressure compensating valve155 (e.g. the pressure may drop in proportion to the amount of reduction in the rotational speed of the engine29).

The rest of the construction of this embodiment is identical to that of the hydraulic system of the embodiment shown inFIGS. 4-5 andFIGS. 10-14.

The construction of the above-described embodiments can be applied to any other self-propelled type work machine, e.g. a work machine such as a backhoe, having a traveling apparatus operated by a hydraulic motor driven by discharge oil from a hydraulic pump driven by an engine, and a work implement hydraulically driven by the discharge oil from a hydraulic pump driven by the engine.

Claims (13)

The invention claimed is:

1. A work machine having a traveling apparatus and a work implement driven by an engine,

wherein a hydraulic system mounted on the work machine comprises:

an HST pump comprising a swash plate, variable displacement pump driven by the engine,

an HST motor connected in a closed circuit to the HST pump and a pair of speed-changing oil passageways, the HST motor being driven by an amount of oil supplied by the HST pump, thereby to drive the traveling apparatus,

a main pump driven by the engine, the main pump supplying pressure oil to the work implement,

a pilot pump driven by the engine,

a swash plate positioning circuit configured to effect positioning of a swash plate of the HST pump with pilot oil supplied from the pilot pump, and

a bleed circuit configured to drain, via a throttle, a portion of the pilot oil supplied to the swash plate positioning circuit.

2. The work machine according toclaim 1, wherein the hydraulic system further comprises:

a charge relief valve for determining the circuit pressure of a charge circuit for supplementing an amount of oil from the pilot pump to the speed-changing oil passageways, and

a flushing throttle incorporated in a flushing relief oil passageway for draining oil from a flushing valve configured to allow escape of a portion of work oil in the low-pressure side oil speed-changing passageway of the speed-changing oil passageways.

3. The work machine according toclaim 2, wherein the flushing relief oil passageway incorporates a flushing relief valve disposed between the flushing valve and the flushing throttle.

4. The work machine according toclaim 1, wherein:

the HST motor comprises a swash plate, variable displacement motor switchable between a first speed state and a second speed state, according to the positioning of the swash plate by pilot oil, and

for switching over the HST motor from the first speed state to the second speed state, there is provided a second bleed circuit for draining, through a throttle, a portion of pilot oil supplied from the pilot pump.

5. The work machine according to clam1, wherein:

the work implement comprises a bucket capable of effecting a scooping/dumping action; and

the hydraulic system includes a bucket cylinder for causing the bucket to effect a scooping/dumping action and a bucket control valve for controlling the bucket cylinder; and

wherein in a hydraulic circuit at a scooping position where the bucket is caused to effect a scooping action by the bucket control valve, there is provided a bucket bleed circuit for draining, through a throttle, a portion of the pressure oil supplied from the main pump to the bucket cylinder.

6. The work machine according toclaim 5, wherein at the scooping position of the bucket control valve, the pressure oil from the main pump is supplied to a rod side oil chamber of the bucket cylinder.

7. The work machine according toclaim 6, wherein:

the bucket control valve includes two pump ports for inputting the pressure oil from the main pump, two cylinder ports for supplying/discharging pressure oil to/from the rod side oil chamber and a bottom side oil chamber of the bucket cylinder, and a tank port communicated to a tank; and

at the scooping position of the bucket control valve, one of the pump ports becomes communicated to the tank port via the bucket bleed circuit and also the other pump port becomes communicated to the cylinder port connected to the rod side oil chamber of the bucket cylinder.

8. The work machine according toclaim 7, wherein the bucket control valve includes a return oil passageway for returning the oil discharged from the bottom side oil chamber of the bucket cylinder to the upstream side of the throttle of the bucket bleed circuit via a check valve, at the scooping position.

9. The work machine according toclaim 5, wherein:

the work implement includes the bucket at a leading end of an arm which is pivoted up/down by a lift cylinder operable by the pressure oil from the main pump; and

the hydraulic system further comprises:

a lift cylinder for pivoting up/down the arm with the pressure oil from the main pump,

an arm control valve for controlling the lift cylinder;

a work implement operating apparatus for operating the arm control valve and the bucket control valve;

pressure reducing means for reducing a primary pressure of the work implement operating apparatus for preventing full stroke action of the arm control valve, in response to drop in rotational speed of the engine;

wherein said arm control valve and said bucket control valve are disposed in series in a work implement supplying oil passageway for supplying the pressure oil discharged from the main pump, with the arm control valve being disposed on the upstream side of the bucket control valve.

10. The work machine according toclaim 1, wherein:

the work implement comprises a bucket capable of effecting a scooping/dumping action; and

the hydraulic system includes a bucket cylinder for causing the bucket to effect a scooping/dumping action and a bucket control valve for controlling the bucket cylinder;

the bucket control valve includes two pump ports for inputting the pressure oil from the main pump, two cylinder ports for supplying/discharging pressure oil to/from a rod side oil chamber and a bottom side oil chamber of the bucket cylinder, and a tank port communicated to a tank;

at the scooping position of the bucket control valve, one of the pump ports becomes communicated to the tank port via the bucket bleed circuit and also the other of the pump ports becomes communicated to the cylinder port connected to the rod side oil chamber of the bucket cylinder.

11. The work machine according toclaim 1, wherein

there is provided a traveling operating apparatus for controlling pilot oil in the swash plate positioning circuit;

there is provided a work implement operating apparatus for controlling the pilot pressure to a control valve for controlling the work implement;

a hydraulic passageway is branched so as to supply the discharge oil from the pilot pump to the traveling operating apparatus and the work implement operating apparatus, and in a hydraulic circuit between this branching point and the traveling operating apparatus, there is provided a pressure compensating valve for ensuring a primary pressure for the work implement operating apparatus, and

on the upstream side of the pressure compensating valve, there is provided a work implement bleed circuit for draining, through a throttle, a portion of the pressure oil discharged by the pilot pump and supplied to the work implement operating apparatus.

12. The work machine according toclaim 1, wherein

there is provided a traveling operating apparatus for controlling pilot oil in the swash plate positioning circuit;

there is provided a work implement operating apparatus for controlling the pilot pressure to a control valve for controlling the work implement;

a hydraulic passageway is branched so as to supply the discharge oil from the pilot pump to the traveling operating apparatus and the work implement operating apparatus, and in a hydraulic circuit between this branching point and the traveling operating apparatus, there is provided a pressure compensating valve for ensuring a primary pressure for the work implement operating apparatus, and

on the upstream side of the work implement operating apparatus, there is provided a pressure reducing valve capable of pressure adjustment so as to allow drop of the primary pressure of the work implement operating apparatus.

13. The work machine according toclaim 1, wherein:

there is provided a traveling operating apparatus for controlling the pilot oil in the swash plate positioning circuit;

the HST pump includes a pair of pressure receiving portions for receiving the pilot pressure from the traveling operating apparatus via a shock relieving throttle, and the angle of the swash plate is controlled by pressure difference between these pressure receiving portions, in the course of which in association with reduction in the rotational speed of the engine, the primary pressure of the traveling operating apparatus drops and the swash plate of the HST pump returns to the neutral side, thus preventing engine stall; and

there is provided an escape oil passageway having one end thereof communicated to a primary side hydraulic passageway of the traveling operating apparatus and the other end thereof communicated to a hydraulic passageway between the shock relieving throttle and one of the pressure receiving portions, and the escape oil passageway incorporates a check valve which is opened when the pressure of the one pressure receiving portion to which the other end of the escape oil passageway is communicated is greater than the primary pressure of the traveling operating apparatus.

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